Flexible printing plates are well known in the graphic arts industry. These plates are usually embodied by flexible material, such as rubber, that has printing elements etched, cut, or otherwise formed, in a surface of the plate. The plate is then mounted onto a drum or other printing platen and ink is applied over the printing elements. Upon bringing the platen and plate in contact with the article surface to be printed on, the ink is transferred to the article.
It is known that printing equipment will not always provide precisely even pressure over the entire printed surface. In fact, with older equipment, pressure on the printing plate is likely to become more inconsistent. With known printing plates, it is common to have smudging or dot gain and halo effects, which reduces resolution. In other situations, the medium that will be printed upon is uneven.
During printing, the flexible printing plate is compressed perpendicular to its printing surface. Depending on the type of material used in the printing plate, this compression can cause lateral spreading of the printing elements, which also can increase the dot size of the printed image (i.e., dot gain). To compensate for some of these deficiencies, manufacturers have added other layers to the printing plate, such as a foam layer. The force of compression is then absorbed by the foam layer, rather than by the printing elements. Behind the foam layer, other layers such as a firm stabilizing layer can also be included as part of the printing plate, providing a solid surface for attachment to the platen.
An ideal material for a flexible printing plate should have certain mechanical properties. In particular, the flexible printing plate should have high abrasion resistance, low resilience, high energy damping and high ink density transfer capability. Such a combination of properties has not yet been obtained in a flexible printing plate and, thus, at least some of these properties are compromised to a certain extent in existing printing plates. The leading printing plate materials, for example, the FP5001 LASERFLEX.RTM. laser engravable rubber plate from Fulflex, Inc., typically have a Shore A hardness of about 55, a Bashore resilience of about 45%, a Taber abrasion loss (3,000 revolutions on a H-22 wheel) of about 0.13 mg/revolution.
Various applications for elastic materials have led to the development of a wide range of natural and synthetic rubbers. Many of the more demanding situations have required blends of these rubbers to provide the proper mix of characteristics. For example, vehicle tires often include styrene-butadiene rubber (SBR), which is the most common synthetic elastomer, polybutadiene (BR), and even natural rubber. The characteristics usually associated with natural rubber, i.e., abrasion resistance, resilience, good high- and low-temperature performance, and tear strength are ideal for tires and similar applications which are subject to extreme conditions.
Other environments which have less demanding strength requirements make other strict demands on elastomers. For example, in the clothing industry, elastomers used for form fitting clothing have a unique set of requirements. These include a low stretch modulus, high dimensional stability, low permanent set, and high tear resistance.
In recent years, a new type of elastomer has become available, namely epoxidized natural rubber (ENR). ENR is usually produced by the chemical modification of natural rubber latex with peroxycarboxylic acids. A key difference in the properties resulting from this modification is increased resistance to swelling by hydrocarbon oils and solvents. ENR also has very high tensile strength and low fatigue properties and can be reinforced to a high degree with silica fillers, even in the absence of a coupling agent.
To benefit from the properties of both natural rubber and epoxidized natural rubber, there have been attempts to form a hybrid material by blending natural rubber and epoxidized natural rubber. However, the specific interactions between the hydrogens of isoprene units (in the natural rubber) and the oxirane oxygens of epoxidized isoprene moieties (in the ENR) are weak and, thus, the two materials do not mix well. Without proper uniformity in the attempted blends, it has been difficult to form a blend that has consistent properties needed for specific applications, e.g., vehicle tires or footwear.
In Japanese patent application 1992-126737, a composition of ENR and natural rubber is disclosed, although large percentages of carbon black and oils are necessary to produce the tire treads disclosed therein.
U.S. Pat. No. 5,447,976, of the present applicant, describes an elastic composite including natural rubber and epoxidized rubber components. This elastic composite has reduced oil swell and absorption, lower permanent set, lower modulus of elasticity and high tear strength, compared to previously known elastic composites of natural rubber and epoxidized natural rubber. The elastic composite disclosed in U.S. Pat. No. 5,447,976 is particularly useful for legbands, straps and contours of swimwear and other garment components.
For the purposes of U.S. Pat. No. 5,447,976, the most suitable composition of natural rubber and epoxidized natural rubber (ENR) includes an amount of ENR which satisfies the equation: ##EQU1##